Well first off, if the 18's have the correct tire size, 205/35/18, then the overall diamter should still be very close to the 17's if you have 205/40/17. Again, if the tire size is correct, then your car won't sit any higher or anything like that....
If the tire size is not 205/35/18, then YES you will have some problems. If your dropped too low, then you will have some rubbing issues. Also if the offset is not 42-45mm, then this will also contribute to rubbing problems.
Overall though, the 18's will look more bling, bling, weigh more (which will kill your stop and go acceleration), and also ride a lot harsher - meaning the bumps and pot holes you were felling before with the 17's will be amplified even more....